Party sources have warned that the changes, intended to clean up the selection of candidates for Westminster, would hit Labour’s finances and face opposition from unions. Unite has given the Labour party £8.4 million since Mr Miliband’s election as leader in 2010.

On Monday night, Len McCluskey, the leader of Unite, indicated that the union would oppose the moves. Mr Miliband’s plan is an attempt to regain the initiative after a week spent fighting allegations of malpractice by the party’s union funders in the selection of Labour candidates for Westminster.

Police are investigating claims that Unite, Britain’s biggest trade union, tried to pack the Falkirk Labour Party with its members to ensure its preferred candidate was chosen to fight the next election.

On Monday the Conservatives called for a police inquiry into two more constituencies where questions have been raised over the processes for selecting candidates.

Unite, which denies wrongdoing, dismissed the call for police to step in as an “obscene” political stunt that will waste detectives’ time.

In his speech, Mr Miliband will promise reforms to eradicate the “bad practices” within his party and in politics more widely. He will say that the allegations over Unite’s activities in Falkirk demonstrated why “machine politics” was “hated — and rightly so”.

He will call for more efforts to “mobilise” the three million who are already affiliated to the party through their unions.

A new code of conduct and strict spending limits will be applied to campaigns to select Labour candidates for Westminster seats, he will say.

The proposals were billed as the most radical party reforms for 20 years. They would lead to a reduction in the £8 million a year that Labour receives in affiliated union membership fees.

But Labour officials could not give a timetable for when the changes will be introduced or what amendments to party rules will be required. A senior Labour figure, to be named on Tuesday, will lead a review of how the reforms can be taken forward.

The plans were immediately criticised by unions. Writing in The Guardian, Mr McCluskey said: “Switching to an 'opt-in’ for the political levy wouldn’t work – it would require Labour to unite with the Tories to change the law, would debilitate unions’ ability to speak for our members and would further undermine unions’ status as voluntary, and self-governing, organisations.”

“Under Ed Miliband’s weak proposals, including a code of conduct that already exists, it would still be the same old Labour Party - bankrolled by the unions, policies rigged by the unions and candidates chosen by the unions,” he said.

Senior figures in the Labour party believe the reforms may take “years” to introduce and there is no guarantee that they will be in place in time for the next general election in 2015.

Labour is also expected to decline an offer from Nick Clegg, the Deputy Prime Minister, to help write a new law to enforce the changes to Labour’s trade union links.

In the Commons on Tuesday, Mr Clegg, the Liberal Democrat leader, is expected to offer Mr Miliband the chance to suggest that a Bill introducing a statutory register of lobbyists could be rewritten to include an “opt-in” system for payments from trade union affiliations.